There is increasing need for new devices that can operate at frequencies greater than 1 GHz. The wide bandgap semiconductor gallium nitride (GaN) has received much interest in the last decade as a material for high efficiency, high power microwave devices due to its high breakdown field and electron saturation velocity, and low thermal generation rate. Unfortunately, there is no native substrate on which to grow GaN device epilayers, hence the films are grown heteroepitaxially on sapphire or silicon carbide substrates.
Such growth results in films with considerable vertical threading dislocation densities that limit electron mobility in lateral devices such as metal semiconductor field effect transistors (MESFETs) or high electron mobility transistors (HEMTs). Yet the vertical mobility is enhanced. This physical attribute together with the material properties previously mentioned makes GaN a highly suitable material for a permeable base transistor (PBT).
The PBT is a device very similar to the MESFET but with vertical transport instead of lateral, and has received much attention in the last quarter of a century due to its potential as a high power, high frequency, and high temperature-operating device. PBTs were suggested as early as 1964, and are typically fabricated using silicon, cobalt disilicide, gallium arsenide, silicon carbide, nickel phthalocyanine, and copper phthalocyanine.
Due to the early stage of development in the GaN growth and understanding of the material properties, PBTs fabricated with GaN have only recently been attempted. Reported modeling simulations of a GaN based PBT with a cut-off frequency (fT) as high as 24.8 GHz and maximum frequency of oscillation (fmax) of 75.2 GHz. Such performance is relatively limited.
What is needed, therefore, techniques for making high power GaN PBTs using GaN.